Electronic oscillators can be used in a variety of applications to provide an oscillating signal at one or more frequencies. For example, oscillators can be used in radio frequency (“RF”) transmitting and receiving systems to provide a carrier signal or to demodulate a received signal. An example of a simple electronic oscillator, also known as a tank circuit, includes an inductor connected in parallel to a capacitor and a voltage source. In a tank circuit, the oscillator frequency or resonant frequency (fo) can be determined using the capacitance (C) and inductance (L) with the following relationship:
  fo  =            1              2        ⁢        π        ⁢                  LC                      .  Other electronic oscillators include a 555 oscillator/timer, Armstrong oscillator, direct digital synthesis oscillator, phase shift oscillator, quartz crystal oscillator, and Resistor/Capacitor (RC) oscillator.
Conventional oscillators often require a drive signal, such as an applied voltage, in order to oscillate. For example, the tank circuit may include an AC voltage source to drive, or cause, the LC circuit to oscillate at its resonant frequency. Typically, the AC voltage source is physically connected to the LC circuit.
Some applications of LC circuits, however, require that the source of the drive signal be disconnected from the LC circuit. In such systems, LC circuits or other oscillators are driven by an electromagnetic signal from a transmitter. An example of such a system is disclosed in U.S. patent application Ser. No. 11/613,645, titled “Communicating with an Implanted Wireless Sensor,” filed Dec. 20, 2006. The devices disclosed in U.S. patent application Ser. No. 11/613,645 include a sensor, such as an LC resonant circuit, implanted in a patient and a system for communicating with, and determining the resonant frequency of, the implanted sensor. The system drives the sensor using an RF burst that is an energizing signal, causing the sensor to oscillate at its resonant frequency and emit a sensor signal. The system then receives the sensor signal and determines the resonant frequency of the sensor and associated sensor characteristics.
Although such systems have been successful in obtaining characteristics associated with a sensor, a need exists for an oscillator that does not require an energizing signal to cause a sensor to oscillate or additional electronics to control and process signal transmission and reception. Furthermore, a need exists for an oscillator that includes components that are not physically connected.